Loss-of-functional mutations in TET2 occur frequently in Myelodysplastic Syndromes (MDS) and Acute Myeloid Leukemia (AML)6, 7, 20, and are mutually exclusive of mutations in IDH1/2, whose aberrant product, 2-hydroxyglutarate, inhibits TET-mediated conversion of 5mC to 5hmC21. The gene discussed is TET2; the disease is myelodysplastic syndrome.